Because it's harder than it looks.
Check out Georgena Terry's series on how to design and build bikes for women. She started the whole "women's specific design" and is still the only one who got it right.
Neither is a solution. Mixte frame are actually worse than a regular bike because the effective top tube is longer. You also have the addition of weight to a bike that is built for a rider who has a lower muscle mass ratio. The bike is both uncomfortable and heavier. That's not really all that conducive to riding enjoyment. Even with the mixte design, the bike is too large for many small riders.